West Nile Virus (WNV) causes infection in the central nervous system (CNS) in several vertebrate animal species. Humans infected with WNV can develop meningitis and encephalitis, and the elderly and immunocompromised are at greatest risk for severe neurological disease and death. New threats of WNV globally and the lack of available treatments warrant studies to develop effective therapeutics and production technologies that can rapidly transfer candidate therapies into the clinical care setting in a cost conscious manner. We recently developed a plant-derived humanized MAb with promising therapeutic potential, with a desired human N-linked glycosylation pattern. This MAb (hE16) binds to a highly conserved epitope on the envelope protein of virtually all isolates worldwide and shows promising post-exposure therapeutic activity. Nonetheless, our studies show that peripheral delivery of hE16 has a limited window of efficacy in rodents: administration of a single dose of hE16 through an intravenous or intraperitoneal route at day 5 post infection or earlier improves survival rates. In comparison, delivery of hE16 directly into the brain at day 6 after infection improved protection against lethal WNV infection in hamsters. Our preliminary data show that a genetically engineered bifunctional MAb variant of hE16 (TfR-Bif) with the potential for enhanced crossing of the blood-brain barrier (BBB) can be expressed in CHO cells or plants. TfR-Bif retains its ability to bind and neutralize WNV, but importantly gains the ability to bind transferrin receptor (TfR) and endocytose into mouse brain endothelial cells. In the R21 phase of this grant, we will use the mouse TfR-specific TfR-Bif as a proof-of-principle in a mouse model of WNV infection. We will test the hypothesis that TfR-Bif can achieve higher levels in the CNS and extends the window of treatment against WNV encephalitis. To address critical safety issues in the brain, we will also identify TfR-Bif glycoforms that offer full bifunctionality with limited or specific Fc receptor (Fc?R) or C1q binding to eliminate potential antibody-dependent enhancement (ADE) of virus infection or pathogenic inflammation. In the R33 phase, we will develop an analogous bifunctional MAb with human therapeutic potential and test if plants provide distinctive advantages in safety, production cost, and scalability for large-scale production under cGMP. In addition to generating novel therapeutic reagents for WNV, this collaborative study will provides a platform for delivery of MAbs to the CNS, which should be applicable to the treatment of other infectious, inflammatory, or neoplastic diseases in the CNS.